It is well known in a turbocharged internal combustion engine equipped with an air dryer that such an engine has a tendency, during unloading of the air compressor, to allow a significant amount of the engine intake air to be diverted through the compressor and dryer and out to the atmosphere when the compressor is in an unloading mode. As a result of this condition there is an inordinate loss of engine power and waste of fuel. Furthermore, during the unloading cycle of the compressor, excessive amounts of oil are caused to pass around the engine piston rings, due to a vacuum or low pressure condition being created above each piston as the latter reciprocates within its respective cylinder without the air intake valve being actuated to an open position.